Metastable long-lasting

Although the situation with melting in two stages appears a little artificial, we ought to remind ourselves that the phase diagram is made up of thermodynamic data alone. In other words, it is possible to see liquid water at 105 °C, but it would be a metastable phase, i.e. it would not last long ... 

In the last chapter we have used the word order without giving it any precise meaning. Most definitions of order involve thermodynamic concepts. Thus, for example, one might say that the most ordered state of a system is the one to which the system tends as the temperature tends to absolute zero. This definition would, however, be of little service in the present context. Most of the systems which we will discuss are remote from thermodynamic equilibrium. This is true both of the films during their preparation and also of the final prepared films. However, these prepared films are in states of metastable equilibrium which are likely to survive for periods long compared with the time taken to carry out experiments on them and, very often, for periods so long as to be, from a human point of view, infinite. 

Smondyrev and Berkowitz [73] performed a rather long simulation (2 ns) of a system with a low cholesterol content (ratio of 1 8 or 11 mol%) and of two systems that contained cholesterol at 1 1 molar ratio and differed in the arrangement of the cholesterol molecules (Figure 6.11). In the simulation with low cholesterol content, the area per lipid was about 62 A2, close to that found in a simulation of pure DPPC for the first approximately 800 ps. However, after this time it started to decrease and reached a value of 58.3 A2 after an additional 500 ps, where it remained for the last 750 ps of the simulation. In the simulation with high cholesterol content, the area per DPPC-cholesterol heterodimer surface area showed also a decrease over the simulation time that was approximately exponential (Table 6.7). In system A (Figure 6.11), with a more uniform distribution of cholesterol, the surface area tended to decrease over the full simulation time of 2 ns, whereas for system B a stable and somewhat lower value was obtained after 1 ns. Although the authors concluded that system A, with more uniformly distributed cholesterol, might be trapped in a metastable state, this system was used in a later publication for comparison of the effects caused by cholesterol sulfate (see below). 

One radioactive isotope of molybdenum is commonly used in medicine, molybdenum-99m. (The m in this instance stands for metastable, which means the isotope does not last very long.) This isotope is not used directly, however. Instead, it is used in hospitals to make another radioactive isotope, technetium-99m. This isotope of technetium (atomic number 43) is widely used as a tracer for diagnostic studies of the brain, liver, spleen, heart, and other organs and body systems. 

However, these molecular dynamics calculations suffer some limitations the empirical nature of the potential (especially for the metal-support interaction) and the arbitrary separation between the metal-metal and metal-support interactions (the metal-metal potential is probably perturbed near the interface). Indeed, according to the type of potential used, very different results are obtained. In the case of Pd/MgO, a mean dilatation [91] or contraction [92] is observed. For finite-temperature molecular dynamics, the calculations are limited to very short times and it is not sure that the equilibrium shape is reached. As we have seen in the last section the cluster shape can be blocked for a long time on facetted metastable shapes. 

During these last 10 years, the number of molecules identified in the interstellar medium has spectacularly increased. Most among the hrmdred of molecules now known have something in common their exotic character from the viewpoint of chemists. Whether they are radicals, carbenes, reactive ions, or metastable isomers of more common species, laboratory studies on such compounds usually remain problematic their reactivity is usually so high that they are either difficult to produce, or a least, difficult to keep alive for a long enough time to record their spectral signatures. 

A last comment will be on metastable species. Atoms and nmlecufes are sometimes excited into sudi electronk states from which a radiative transition is not allowed by quantum mechanical rules. Atoms and molecules in these so-called metastable states may have very long lifetimes and consideraUy influence the overall discharge chemistry. A metastable atom (molecule) can transfer its energy throu a collision with another particle and, if this is of lower ionization potential, the result may be an ionization or dissociative ionization event Such processes are known as Praning ionization (nrocesses and may be in rtant under cotain conditions in plasma chemistry. 

More recently, a closer inspection of the transient stress rheology for thickening systems has revealed more complicated patterns, such as structural memory effects. Berret et al. [78] and Oeschlager et al. [93,107] have observed that the transient mechanical response also depended on the thermal and shear histories. Samples having been treated thermally, e.g., heated up to 90 °C for 2h, behaved very differently from samples freshly prepared or already sheared. The induction time couid last several hours, and was not proportional to the inverse shear rate, as mentioned previously. It was concluded that the lack of reproducibility under certain thermal and shear conditions might indicate that these surfactant solutions were characterized by long-lived metastable states. 

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